In the field of residential and/or consumer lighting, there have been significant efforts made in the past several years to encourage more widespread public use of a compact fluorescent lamps in place of less efficient incandescent lamps. Because of the significant energy savings that a fluorescent lamp offers over the use of an incandescent lamp while still attaining a comparable level of light output, public acceptance of such a lamp could contribute to the overall aim of conserving energy and the natural resources that are used to generate such energy. It is also a significant advantage that such compact fluorescent lamps have a considerably longer life than a conventional incandescent lamp. To this end, compact fluorescent lamps have been introduced having a standard type of lamp base so that they may be inserted into a typical lamp socket. An example of such a compact fluorescent lamp can be found in U.S. Pat. No. 4,503,360 issued on Mar. 5, 1985 to D. E. Bedel. Although the lamp of this patent is an electroded compact fluorescent lamp, it is possible to achieve an even longer lamp life using an electrodeless fluorescent lamp as described in U.S. Pat. No. 4,010,400 issued to Hollister on Mar. 1, 1977.
As with most fluorescent or low pressure discharge lamp devices, it is necessary to provide a ballasting circuit to perform the function of conditioning the current signal used to drive the discharge lamp. Examples of typical ballasting circuits for a compact fluorescent lamp can be found in U.S. Pat. Nos. 4,443,778 and 4,570,105 issued respectively on Apr. 17, 1984 to J. A. C. Mewissen and Feb. 11, 1986 to H. J. Engel. It will be noted that the ballasting circuits described in each of these patents relies on an electromagnetic type of ballasting; that is, one that requires the use of a magnetic core transformer to condition the current signal. Because such a ballasting arrangement operates at a power line current frequency of 60 hz which can result in lamp flicker, it has been determined that an electronic high frequency ballast that would eliminate the occurrence of lamp flicker or light variation, would be preferable. An example of a high frequency electronic ballast arrangement for a gas discharge lamp can be found in U.S. Pat. No. 4,546,290 issued to B. Kerekes on Oct. 8, 1985. An example of a circuit for an electrodeless fluorescent product can be found in U.S. Pat. No. 4,383,203 issued to Stanley on May 10, 1983. It will be noted that typical electronic circuits of this type are of a self-resonant variety wherein a sample of the resonant current is taken to signal the switching of the bipolar transistor. Though this arrangement has proven to be simple and reliable in some applications, its efficiency at operating frequencies above 20 kHz is less desirable since the dynamic losses in the switching devices can be prohibitive. The term "dynamic losses" can be considered as that amount of energy in the circuit that is diverted away from the energization of the lamp and is otherwise lost in the form of heat dissipated. As such, if dynamic losses are not kept in check, the switching devices could experience a thermal runaway condition or operate at such a high temperature that reliability guidelines are exceeded. U.S. Pat. No. 4,988,920 issued on Jan. 29, 1991 to G. S. Hoeksma illustrates a more recent example of an electronic ballast circuit for a typical fluorescent lamp. In this patent, semiconductor switching devices such as metal oxide semiconductor field effect transistors (MOSFETs) are utilized to achieve the necessary switching frequency that allows the ballast to operate at a higher frequency than does a typical electromagnetic ballast arrangement or even electronic ballast which uses bipolar transistors for switching. It has been found that MOSFET devices offer advantages over the bipolar transistor approach and that such MOSFETs can also be reliably operated at yet a higher frequency than the bipolar devices. In addition to the advantage that an electronic ballast will weigh significantly less than its magnetic counterpart, the electronic ballast, by operating at the higher frequency, provides for a higher light output, avoids the problem of light flicker and further reduces light output variation relative to a ballast operating at 60 hz.
One of the inherent considerations in the design of the compact fluorescent lamp and an electronic circuit for driving such compact fluorescent lamp is the avoidance of generating electromagnetic interference (EMI) in a particular frequency range established by the U.S. Federal Communications Commission (FCC) as being between 400 kHz and 30 mHz, a range that can be referred to as the compliance band. An example of a lamp ballasting arrangement using MOSFETs and operating at a high frequency can be found in Application Note AN-973 entitled "HEXFETs, Improve Efficiency, Expand Life of Electronic Lighting Ballasts" published by International Rectifier of El Segunda, Calif. A circuit embodying the discussion of this publication is provided herewith as FIG. 1 which will be described hereinafter in further detail. It is sufficient to note at this time however that though the circuit of this approach avoids the problem of increased dynamic losses by increasing the switching speed, such increased switching speed results in an increased EMI which may fall within the previously stated compliance band. One approach to avoid emitting EMI in the compliance band is to utilize an interference shielding device such as a filter. Such devices however are bulky and costly thereby adding to the weight, size and/or cost of the discharge lamp. Therefore, it would be advantageous if one were to provide an electronic ballasting circuit for a compact fluorescent lamp that would operate without experiencing dynamic losses that could affect performance and life expectancy yet achieve this improved performance without creating significant amounts of EMI.